Zero gravity operation of various equipment requires modifications of its conventional counterparts. For example, in a gravity environment, heat can be removed from a gaseous stream with a conventional condensing heat exchanger (hereinafter referred to as condenser). Upon the removal of the heat, a condensate forms and drains to the bottom of the condenser. However, in zero gravity, this condensate will not automatically drain from the condenser since there are no gravitational forces drawing the condensate to the bottom. Instead, the condensate will form droplets which will block the gaseous flow through portions of the condenser. As a result, the condensate droplets can be forced out of the condenser by the gaseous flow in what is referred to as condensate "carry over". Therefore, additional equipment is required to remove the condensate from the condenser.
In order to minimize this "carry over" in zero gravity, many condenser heat transfer surfaces are coated with a hydrophilic coating. This hydrophilic coating causes wetting and wicking, thereby inducing condensate in the condenser to form a thin spreading film in the coating which can readily be collected. This thin film is collected through "slurper" holes into a gas-liquid phase separator which keeps water droplets from being entrapped in the gaseous stream from which it was removed. (see U.S. Pat. No. 3,868,830.)
U.S. Patent No. 3,658,581 (incorporated by reference) to Paul et al. discloses a hydrophilic passive coating which facilitates wetting and wicking on heat transfer surfaces. The hydrophilic qualities of this passive coating result from the chemical polarity of uncoated silica or calcium silicate dispersed in a non-crystalline binder and from the capillary attraction of the water molecules for one another. The silica and calcium silicate particles have a polar attraction to hydroxyl ions in the condensate water and thereby pull the water to the coating, known as wetting. Wicking or capillary attraction then comes into play as the water being drawn into the coating pulls additional water along with it.
Due to the porous characteristic of this hydrophilic coating, however, it can potentially entrap organic, inorganic and microbial contaminants. As a result, the coated heat transfer surfaces constitute ideal locations for microbial proliferation which can reduce the hydrophilic properties of the coating, plug slurper holes, and corrode the heat transfer surfaces, thereby decreasing the heat transfer efficiency of the condenser. Additionally, if these microbes become air borne, they can be inhaled and cause adverse health effects and they can result in odor generation in the gaseous stream exiting the condenser. As a result, microbial proliferation constitutes heat transfer efficiency, health, and comfort concerns in relation to condensers, especially condensers which operate within a closed environment.
Since the coating disclosed in Paul et al. has generally only been utilized for about 7 to 10 consecutive days, microbial proliferation has not been a great concern. After use, these condensers and the heat transfer surfaces would dry, thereby inhibiting microbial proliferation. However, in applications where the condenser will be utilized for extended periods of time, such as on a space station for 10 years or more, microbial proliferation becomes a major concern. Consequently, what is needed in the art is an antimicrobial additive for the hydrophilic coating used on heat transfer surfaces which does not adversely affect wetting and wicking characteristics.